Acrylic compositions are completely different, in terms of their structure and properties from linear (thermoplastic) and so-called network (cross-linked) acrylic polymers produced from high-molecular weight polyacrylates or from monomer and oligomer acrylates. Despite the fact that acrylic compositions per se are well known, nevertheless they are not used for sealing purposes as a foamable one-component product, see e.g. Edward M. Petrie, Handbook of Adhesives and Sealants, N.Y. 2000, pp.390-392. In the present specification by the one-component product is meant a composition, that contains all its ingredients within a single package (e.g. a can), and its foaming and polymerization is caused by exposing the content of the package to ambient air.
In practice various polyurethane compositions are used as such one-component foam sealants. Polyurethane one-component foam compositions have good service properties but their use is relatively limited, since they contain very toxic isocyanate components. Polyurethane sealants are especially harmful in those applications in which the sealant is applied manually, see e.g. Robert M. Evans, Polyurethane Sealants. 1993, Technonic Publishing Company, N.Y. 1993.
There are known also silicone based one-component foam sealants, however they are very expensive and in many applications their mechanical properties are not sufficient for obtaining reliable sealing, see e.g. Eduard M. Petrie, Handbook of Adhesives and Sealants, N.Y. 2000, pp. 497-498.
In practice it would be very convenient if a one-component sealant were curable by virtue of photo-polymerization induced by exposing the sealant to ambient air or to a UV or IR laser radiation.
So far, this measure has not been exploited for polymerization of foamable acrylic sealants but merely for acrylic compositions used as coatings.
A process for producing of rigid acrylic foam is disclosed in U.S. Pat. No. 5,026,738 to Stefan. According to this patent it is possible to obtain rigid foam having bulk density ranging from 50 to 100 g/l by polymerizing and foaming of acrylic acid and/or methacrylic acid compounds in the presence of catalysts and/or initiators liberating nitrogen. The polymerization process is carried out at elevated temperature and upon completing thereof the obtained foam is evacuated from a mold. Unfortunately the particular compositions mentioned in this patent are not suitable for photo-polymerization by a UV or IR laser irradiation.
In JP6073226 and JP6128407 to Yoshiaki, et al. are disclosed acrylic foam compositions based on emulsion of a (meth)acrylic ester polymer. The foamed compositions are used as soundproofing or cushioning materials. The composition disclosed in JP6073226 contains 10-99,9 wt. % of (meth)acrylic ester, 0.1-10 wt. % of a monomer containing a cross linking functional group and 0-89 wt % of a monomer which gets free from the crosslinked functional group. The compositions described in the above patents are polymerized when an emulsion of a (meth)acrylic ester polymer is prepared by mixing with water at 10-90 deg. C. for 5-15 hours in the presence of an emulsifier and a polymerization initiator. The obtained emulsion is mixed with 0.05-10 weight parts of Fluorosilicate and then is heated to 80 deg. C. for 10 minutes or longer to obtain a crosslinked foam. Unfortunately these acrylic foamable compositions are not suitable for sealing applications since they are defined by an open surface, which is required for fast and easy removal of water.
In International publication WO 0204200 to Ramesh is described pressure-sensitive adhesive foam-like tape having a backing comprising 88-92% of an acrylic polymer. This polymer contains: 35-45% of a first acrylate monomer wherein the alkyl groups contain from 4 to 12 carbon atoms, 30-40% of a second alkyl acrylate monomer of analogous structure, 6-10% of a first monoethylencally unsaturated polar copolymerizable monomer, 1-2% of a second monoethylencally unsaturated polar copolymerizable monomer, 0.3-0.5% of a photoinitiator, 1-2% of fillers, 0.05-0.07% of a crosslinker/chain extender, and 8-12 of hollow glass microspheres dispersed evenly in the polymer. The tape further comprises a pressure-sensitive adhesive layer covering at least one side of the backing. Unfortunately this composition is not suitable for use as a foam sealant, since it is curable only under pressure in the form of a thin film.
Thus despite various polymerizable compositions are known for sealing applications, nevertheless there is still felt a strong need in a new, one-component, acrylic-based, foamable composition.